The term "mineral" as used herein means a natural or a synthetic inorganic solid that is insoluble in water and that has a high melting point. The porous crystalline minerals referred to herein melt at temperatures usually in excess of 750.degree. C., and some retain their porosity and crystallinity even after calcination at 1000.degree. C.
The crystalline minerals referred to herein have highly ordered, robust three-dimensional framework structures as evidenced by well-defined and reproducible X-ray diffraction patterns which are distinctive for the different framework structures. The ordered structures contain intracrystalline micropores, i.e. pores of molecular dimensions, regularly disposed in the crystal lattice and readily distinguishable from the much large extracrystalline pores formed by agglomerates of microcrystals. Because of their intracrystalline nature, these pores are very uniform and, when free of occluded matter, selectively sorb only those molecules having a critical diameter that can be accomodated by the pore size of the particular mineral in question. Such microporous minerals are often referred to as "molecular sieves". The terms "pores" and "porous", as used herein, refers to the intracrystalline micropores unless explicitely stated to be otherwise.
The three-dimensional frameworks of the crystalline microporous minerals useful herein are formed by one or more tetrahedrally bonded elements linked together by covalent bonds to oxygen atoms. Such structures are found in nature as aluminosilicate deposits such as erionite and mordenite. These naturally-occurring robust structures are not electoneutral because of the tetrahedrally incorporated trivalent aluminum, and as a result the structures must be associated with hydrogen cations and/or metallic cations. These cations are contained in the micropores of the crystal, and usually may be ion exchanged with other cations. For purposes of the present invention, the term "framework" as used herein is intended to refer only to the tetrahedrally bonded element or elements together with the associated oxygen of the robust framework, and to exclude the mobile cations that may be present. For a more detailed description of such microporous minerals, the reader is referred to "Zeolite Molecular Sieves" by D. W. Breck, Wiley, N.Y., 1974, the content of which is incorporated by reference for background.
The art of mineralizing inorganic gels to produce synthetic microporous crystalline minerals of the type described herein has advanced considerably in recent years. The earliest developments provided not only counterparts of naturally occurring aluminosilicate minerals such as faujasite, but also of aluminosilicate minerals not found in nature, such as Linde Zeolite A. In general, the frameworks of these early synthetics and of the naturally occurring zeolites were relatively rich in alumina, having a silica to alumina molar ratio not greater than about 10. Later developments, involving the inclusion of organic templating agents in the mineralizing compositions, led to minerals having no natural counterparts and framework compositions very rich in silica, as well as framework compositions in which the tetrahedrally bonded elements are aluminum and phosphorous, i.e. having no silicon at all. In addition, such structures have been synthesized which contain gallium, iron, and boron as a tetrahedrally bonded element. The descriptive phase "microporous crystalline mineral" as used herein is intended to mean a natural or synthetic mineral having a robust three-dimensional structure of the type described above and associated cations, if any are present, regardless of the indentity of the tetrahedrally bonded elements unless these are explicitely specified. Furthermore, for purposes of clarity and concise expression, any reference made herein in the disclosure and in the claims to the "framework composition" is intended to refer to the tetrahedrally bonded elements of the framework.
Certain microporous crystalline minerals have found extensive use as catalysts in the processing of petroleum, in the production of petrochemicals, and to a lesser extent in the chemical industry. Catalytic cracking, hydrocracking, catalytic dewaxing, alkylation, dealkylation, transalkylation, isomerization, polymerization, additions, disproportionation, conversion of methanol to hydrocarbons, and other acid catalyzed reactions may be performed with the aid of such catalysts.
It has long been known that the content of mobile cations in a microporous crystalline mineral is that which stoichiometrically balances the electrostatic charge of the framework composition, and in general the cation composition itself may be reversibly changed by ion-exchange. The composition of the robust framework, once formed, however is not so readily altered. Very recently issued patents, such as U.S. Pat. No. 4,513,091 to Chang et al. describes a hydrothermal method for introducing tetrahedrally bound aluminum into the structure of a high silica content crystalline zeolite having a silica to alumina ratio of at least 70:1 and a Constraint Index of 1 to 12. U.S. patent application Ser. No. 493,192 filed May 13, 1983 now U.S. Pat. No. 4,576,805 describes a method for enhancing the catalytic activity of a zeolite by contact at high temperature with a volatile compound to be coordinated in the framework of the zeolite.
The use of molten salt to occlude the salt in a crystalline zeolite has been described by Barrer and Meier, J. Chem. Soc., p. 299 (1958). However, there appears to be no description of the use of a molten salt medium to alter the framework composition of a preformed microporous crystalline mineral.
It is an object of this invention to provide a facile method for altering the composition of the robust framework of a microporous crystalline mineral. It is a further object of this invention to provide a method for altering the catalytic behavior of a microporous crystalline mineral. It is another object of this invention to provide a novel method for increasing the acid catalytic activity of a microporous crystalline mineral having a substantially electroneutral framework. These and other objects of this invention will become evident on reading this entire specification, including the appended claims.